FIG. 1 depicts a chart 10 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 10, the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles.
Various lithium-based chemistries have been investigated for use in various applications including in vehicles. FIG. 2 depicts a chart 20 which identifies the specific energy and energy density of various lithium-based chemistries. In the chart 20, only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. As is evident from the chart 20, lithium/air batteries, even allowing for packaging weight, are capable of providing a specific energy >600 Wh/kg and thus have the potential to enable driving ranges of electric vehicles of more than 300 miles without recharging, at a similar cost to typical lithium ion batteries. While lithium/air cells have been demonstrated in controlled laboratory environments, a number of issues remain before full commercial introduction of a lithium/air cell is viable as discussed further below.
A typical lithium/air electrochemical cell 50 is depicted in FIG. 3. The cell 50 includes a negative electrode 52, a positive electrode 54, a porous separator 56, and a current collector 58. The negative electrode 52 is typically metallic lithium. The positive electrode 54 includes carbon particles such as particles 60 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 62. An electrolyte solution 64 containing a salt such at LiPF6 dissolved in an organic solvent such as dimethyl ether or CH3CN permeates both the porous separator 56 and the positive electrode 54. The LiPF6 provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell 50 to allow a high power.
A portion of the positive electrode 52 is enclosed by a barrier 66. The barrier 66 in FIG. 3 is configured to allow oxygen from an external source 68 to enter the positive electrode 54. The wetting properties of the positive electrode 54 prevent the electrolyte 64 from leaking out of the positive electrode 54. Oxygen from the external source 68 enters the positive electrode 54 through the barrier 66 while the cell 50 discharges and oxygen exits the positive electrode 54 through the barrier 66 as the cell 50 is charged. In operation, as the cell 50 discharges, oxygen and lithium ions are believed to combine to form a discharge product Li2O2 or Li2O in accordance with the following relationship:
      Li    ↔                  Li        +            +                        e          -                ⁢                                  ⁢                  (                      negative            ⁢                                                  ⁢            electrode                    )                                        1        2            ⁢              O        2              +          2      ⁢                          ⁢              Li        +              +          2      ⁢                          ⁢                        e          -                ⁢                  ⟷          catalyst                ⁢                  Li          2                    ⁢      O      ⁢                          ⁢              (                  positive          ⁢                                          ⁢          electrode                )                        O      2        +          2      ⁢                          ⁢              Li        +              +          2      ⁢                          ⁢                        e          -                ⁢                  ⟷          catalyst                ⁢                  Li          2                    ⁢              O        2            ⁢                          ⁢              (                  positive          ⁢                                          ⁢          electrode                )            
The positive electrode 54 in a typical cell 50 is a lightweight, electrically conductive (˜Ωcm) material which has a porosity of greater than 80% to allow the formation and deposition/storage of Li2O2 in the cathode volume. The ability to deposit the Li2O2 directly determines the maximum capacity of the cell. In order to realize a battery system with a specific energy of 600 Wh/kg or greater, a plate with a thickness of 100 μm must have a capacity of about 20 mAh/cm2.
Materials which provide the needed porosity include carbon black, graphite, carbon fibers, and carbon nanotubes. There is evidence that each of these carbon structures undergo an oxidation process during charging of the cell, due at least in part to the harsh environment in the cell (pure oxygen, superoxide and peroxide ions, formation of solid lithium peroxide on the cathode surface, and electrochemical oxidation potentials of >3V (vs. Li/Li+)).
Some lithium/air cells contain gas-diffusion electrodes based on barriers 66 made from porous carbon materials like carbon black, graphite, graphene, carbon fibers or carbon nanotubes. The cells are typically operated with pure oxygen. For practical applications one main challenge is the gas supply, because gas cylinders containing pure oxygen will probably not be carried in electric vehicles due to safety reasons.
State-of-the-art gas separation membranes are typically based on polymers, zeolites or molecular sieves which are not 100% gas selective. In order to allow high cycle life (>>1000) of a lithium air cell the concentration of relevant contaminants (i.e. CO2, H2O, N2) in the supplied oxygen must be below 10 ppm. Therefore a gas separation technique is needed which is almost 100% selective regarding oxygen vs. air.
The needed selectivity may be realized by solids which are highly anion-selective due to a distinct solid-state transport mechanism. Examples of this type of material are complex transition metal oxides with oxygen vacancies like La0.6 Sr0.4CoO3-d and Ba0.5Sr0.5Co0.8Fe0.2O3-d. Typically these materials can only be processed as membranes in thin-film processes with a thickness between ˜0.5 and 2 mm. In order to allow anion transport through those membranes one needs to apply high temperatures up to 1300 K. Furthermore the absolute anion-current is limited by the relatively large thickness of the membranes and by the limited rate of surface ionization of oxygen, which is required to form the transportable anion.
In addition to the foregoing difficulties, the oxygen separation layer has to allow a sufficient gas flow (min 5 μl/s·cm2) in order to supply the cell with enough oxygen for typical current densities of about 40 mA/cm2. Gas separation layers based on polymer membranes and silicon oil infiltrated porous structures are known and mainly used for air-water separation. These membranes are very thick (several microns) and do not allow high gas flow rates, which limits the maximum discharge power when used for metal-air batteries. Pressure swing adsorption is an engineering solution to separate N2 from air. This technology requires a power supply and is therefore not suitable to be incorporated in a battery. All these technologies are not able to produce oxygen with contaminants below 10 ppm, which is crucial for a reversible operation of the lithium/oxygen or metal/oxygen battery.
What is needed therefore is a barrier which can separate oxygen from air. A further need exists for a barrier that allows a sufficient amount of oxygen to be introduced into the battery.